1. Field of the Invention
The present invention relates generally to a light reception output controlling apparatus of a photo detector in an optical pickup unit that is configured to guide a laser beam reflected and returned from a disk to the photo detector including a light reception region constituted by a plurality of segments and, more particularly, to a light reception output controlling apparatus of a photo detector in an optical pickup unit that takes measures against an output difference in each light reception output from the photo detector, which is caused by displacement of a light reception spot projected on the light reception region of the photo detector.
2. Description of the Related Art
In an optical pickup unit that optically records and reproduces signals with the use of laser beams to an optical disk such as CD and DVD, an astigmatic method and an application thereof, i.e., a differential astigmatic method are widely used for the focus control that focuses a laser beam irradiated to disk on a signal layer of the disk.
On the other hand, for the tracking control that makes the laser beam irradiated to disk follow a signal track of the disk, one of a three-beam method, a push-pull method, a phase difference method, and applications thereof is employed correspondingly to a disk standard.
For example, an actual example of an optical pickup unit corresponding to various record/reproduction CD as well as DVD-ROM and DVD±R/RW employs the astigmatic method for the various record/reproduction CD and the differential astigmatic method, which is an application of the astigmatic method, for various record/reproduction DVD with regard to the focus control mode and employs the phase difference method for DVD-ROM and a differential push-pull method, which is an application of the push-pull method, for DVD±R/RW and CD with regard to the tracking control mode.
When a basic version and an applied version of the astigmatic method is employed in the focus control, the photo detector includes an astigmatism generation optical system such as an anamorphic lens that generates astigmatism in the reflected laser beam from the disk and a parallel plate disposed in a tilted manner relative to the light axis as well as a light reception region divided into four parts with two orthogonal dividing line forming an angle of 45 degrees relative to the generation direction of the astigmatism.
On the other hand, since three beams are needed for the laser beams irradiated to the disk in the differential astigmatic method of the focus control mode and the differential push-pull method or three-beam method of the tracking control mode, an optical pickup unit employing these modes includes a diffraction grating for diffracting and dividing the laser beam emitted from a semiconductor laser into three beams that are 0th-order light and ±1st-order diffracted light and uses a photo detector including three light reception regions that receives three reflected laser beams, which are these three beams reflected by the disk, as is well known.
By the way, when the optical pickup unit is assembled, the photo detector is positioned and attached to an optical housing disposed with optical devices of the optical pickup unit. See Japanese Patent Application Laid-Open Publication No. 2005-71458.
In the positioning of the photo detector, to obtain appropriate light reception output from each segment constituting the light reception region when a laser spot projected on the light reception region of the photo detector is correctly disposed, a focus error signal and a tracking error signal generated from each light reception output with predetermined calculations is allowed to have S-shaped curve characteristics with suitable symmetric property and amplitude relative to a focus servo and racking servo, and a wobble signal with suitable amplitude is obtained from wobble formed in the DVD recording disk or CD recording disk.
However, although the photo detector is positioned to the optical housing, the laser spot may not be projected correctly on the light reception region of the photo detector because of attachment errors, remaining stress at the time of the attachment, or an application amount output difference and changes over time of the used adhesive, and it is problematic that the focus servo characteristics and tracking servo characteristics may be deteriorated and the amplitude of the wobble signal may be reduced.
In the optical pickup unit employing the differential astigmatic method for the focus control, to take measures to the above problem, it is known to use an offset correction apparatus that corrects an offset based on the position of the light reception spot projected on the light reception region by adjusting a level of a predetermined light reception output relating to the focus error signal such that each light reception output of the four-divided light reception region in the light reception state when the focus error signal becomes “0” (Japanese Patent Application Laid-Open Publication No. 2002-32924).
By the way, in the offset correction apparatus shown in Japanese Patent Application Laid-Open Publication No. 2002-32924, the focus error signal is used, and a configuration including a focus error signal generation circuit that generates the focus error signal by calculating each light reception output of the light reception region of the photo detector, is considered. Since the focus error signal generation circuit is typically built into a disc drive and an apparatus disposed with the focus error signal generation circuit is not shown, the offset correction apparatus is not considered to be self-contained in a single optical pickup unit.
Therefore, unless the disk drive is built into the optical pickup unit, the offset correction is not performed for each light reception output of the four-divided light reception region, and this is difficult to achieve since manufacturers of the optical pickup unit and the disk drive are generally independent from each other.
Although the offset correction apparatus can correct inequality of each light reception output on each diagonal line in the four-divided light reception region generating the focus error signal since the focus error signal is used, inequality is not necessarily corrected in adjacent light reception outputs divided by each dividing line orthogonal to each other in the four-divided light reception region.
Therefore, if the inequality of each light reception output of the four-divided light reception region is corrected by the method using the focus error signal, sufficient effects may not be obtained in improving the amplitude characteristics of the wobble signal or the tracking servo characteristics when the employed tracking control method is the push-pull method or the differential push-pull method that is the application thereof.